Cyclic AMP and cyclic GMP have been implicated as intracellular mediators of erythropoietin (ESF) at target organ sites such as the bone marrow. However, the precise mechanism by which this action occurs is not known. The objectives of the research proposed in this application include an evaluation of the possible roles of these cyclic nucleotides in mediating the effect of ESF in target organs. To achieve this objective, fetal liver cultures are being used as a model system. The effects of ESF on cyclic nucleotide levels and correlations between the accumulation of these cyclic nucleotides and the synthesis of RNA and DNA will be studied at various time intervals following treatment. The direct effects of cyclic nucleotides on DNA synthesis and RNA synthesis will be monitored in synchronized cultures. The effects of cyclic GMP on the progression of the cell cycle will be studied with particular emphasis on the hastening of cells into mitosis. Since the importance of histone phosphorylation in the progression of the cell cycle has been well established, the enzyme systems controlling phosphorylation will be examined. The effects of ESF on cyclic nucleotide dependent protein kinase activities and a characterization of these enzymes and their substrates will be attempted. In addition, dephosphorylation processes will also be monitored using enzyme preparations from erythropoietically stimulated fetal liver.